Is OSGi fundamentally incompatible with JSR-223 Sc

Question

I've recently written a small specialist scripting language and used the Maven to export an OSGi compliant bundle that also exports a service descriptor into the "META-INF/services/javax.script.ScriptEngineFactory" service registry file.

The problem is that although the OSGi import and export packages are fine, the service registry appears to be incompatible with OSGi (as OSGi keeps its bundles off the general classpath and uses separate classloaders for modules).

My question is, am I correct in thinking that OSGi is incompatible with the Service Discovery mechanism, and if not, what can I add to my bundle metadata so that ScriptEngineManager.getEngineFactories() will list my script engine in an OSGi environment?

Answer 1

Apache Sling does use this mechanism in an OSGi environment to manage its JSR-233 compatible script engines, mostly via its ScriptEngineManagerFactory class [1]. See also [2] for an example custom script engine.

Adding your script engine to Sling should work if it's JSR-233 compatible. The simplest way to test that is probably to follow the "Sling in 15 minutes" tutorial [3] using your language instead of the server-side javascript language that's used there.

[1] http://svn.apache.org/repos/asf/sling/trunk/bundles/scripting/core/src/main/java/org/apache/sling/scripting/core/impl/ScriptEngineManagerFactory.java

[2] http://svn.apache.org/repos/asf/sling/trunk/bundles/scripting/javascript

[3] http://sling.apache.org/site/discover-sling-in-15-minutes.html

Answer 2

Matt F. blogged about an alternative solution [1]

When providing scripting in a Java application, scripting engines conforming to JSR 223 (e.g. Groovy, JRuby, Scala, ...) can easily be embedded using something along the lines of

ScriptEngineManager scriptEngineManager = new ScriptEngineManager();
ScriptEngine scriptEngine = scriptEngineManager.getByName("groovy");

However, in an OSGi-based application, the ScriptEngineManager fails to discover scripting engines located in installed bundles, due to the way it discovers engines available on the class path. Luckily, the Apache Felix project has already solved this problem, there are

• OSGiScriptEngineManager [2]
• OSGiScriptEngineFactory [3]
• OSGiScriptEngine [4]

which provide an OSGi-compliant way to discover and load scripting engines installed as OSGi bundles.

ScriptEngineManager scriptEngineManager = new OSGiScriptEngineManager(bundleContext);
ScriptEngine scriptEngine = scriptEngineManager.getByName("groovy");

Now that we've had several years of scripting and OSGi experience, one challenge is simplifying script access to OSGi services. Using the ServiceTracker api [5] seems to be the only approach; but this effort is high for simple scripts. We've worked towards a means for scripts to express which OSGi services they want and automate the ServiceTracker calls on behalf of the scripts but it is fragile. Looking forward to the OSGi specification offering better support in the future.

[1] http://devnotesblog.wordpress.com/2011/09/07/scripting-using-jsr-223-in-an-osgi-environment/

[2] http://svn.apache.org/repos/asf/felix/trunk/mishell/src/main/java/org/apache/felix/mishell/OSGiScriptEngineManager.java

[3] http://svn.apache.org/repos/asf/felix/trunk/mishell/src/main/java/org/apache/felix/mishell/OSGiScriptEngineFactory.java

[4] http://svn.apache.org/repos/asf/felix/trunk/mishell/src/main/java/org/apache/felix/mishell/OSGiScriptEngine.java

[5] https://osgi.org/javadoc/osgi.core/7.0.0/org/osgi/util/tracker/ServiceTracker.html

Answer 3

Just to chime in that there is a standard general solution to consuming these types of services in an OSGi environment, as ScriptEngineFactory is not the only such case. It is a part of the OSGi Enterprise spec. and the reference implementation can be found here: http://aries.apache.org/modules/spi-fly.html

It is trivial to recreate the functionality of the Apache Spring classes via this mechanism, and imo this method is far cleaner and more sensible, but I do understand the desire to avoid reimplementing the wheel so to speak.